We propose a curved triangular patch that is suitable for the field calculation by the integral equation methods. The interior control point of this patch is variably represented in the area coordinate as a weighted average of six control points. We can connect the patch to the three surrounding patches with G 1 continuity on the sides. The whole surfaces to be treated can be represented as a collection of smoothly connected patches. This patch is applied to the surface charge method with the pseudo-particle fast multipole method. We demonstrate some electrostatic field calculations using several hundreds of thousands of proposed patches.